iommu/vt-d: Use cmpxchg16b to update posted format IRTE atomically

If IRTE is in posted format, the 'pda' field goes across the 64-bit
boundary, we need use cmpxchg16b to atomically update it. We only
expose posted-interrupt when X86_FEATURE_CX16 is supported and use
to update it atomically.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>